FIG. 2 illustrates a circuit configuration of a multiple-output DC conversion apparatus according to a related art. FIG. 1 illustrates, as an example of a basic circuit to explain the multiple-output DC conversion apparatus, a circuit configuration of a single-output DC conversion apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-319650.
The DC conversion apparatus illustrated in FIG. 1 consists of a half-bridge circuit. Ends of a DC power source Mm are connected to a series circuit that includes a switching element Q1 made of a MOSFET and a switching element Q2 made of a MOSFET. A drain of the switching element Q2 is connected to a positive electrode of the DC power source Mm and a source of the switching element Q1 is connected to a negative electrode of the DC power source Vin.
Between the drain and source of the switching element Q1, there are connected a diode D1 and a voltage resonant capacitor Crv that are connected in parallel, as well as a series circuit including a reactor Lr1, a primary winding P1 of a transformer T1, and a current resonant capacitor Cri. The reactor Lr1 is a leakage inductance between the primary and secondary sides of the transformer T1. The primary winding P1 is connected to an equivalent reactor Lp1 which is an exciting inductance. Between the drain and source of the switching element Q2, there is connected a diode D2 in parallel.
A winding start of each winding of the transformer T1 is depicted by a filled circle. A first end (with a filled circle) of a secondary winding S1 of the transformer T1 is connected to an anode of a diode D3. A second end of the secondary winding S1 of the transformer T1 and a first end (with a filled circle) of a secondary winding S2 of the transformer T1 are connected to a first end of a smoothing capacitor Co1. A second end of the secondary winding S2 of the transformer T1 is connected to an anode of a diode D4. A cathode of the diode D3 and a cathode of the diode D4 are connected to a second end of the capacitor Co1. The ends of the capacitor Co1 are connected to a load Ro1.
A control circuit 10 alternately turns on/off the switching elements Q1 and Q2 according to an output voltage Vo from the capacitor Co1, to carry out PFM control (frequency control) so that the output voltage Vo from the capacitor Co1 is kept constant.
Operation of the DC conversion apparatus with the above-mentioned configuration will be explained in detail with reference to a timing chart illustrated in FIG. 4.
In FIG. 4, VQ1 is a drain-source voltage of the switching element Q1, IQ1 is a drain current of the switching element Q1, VQ2 is a drain-source voltage of the switching element Q2, IQ2 is a drain current of the switching element Q2, VCri is a terminal voltage of the current resonant capacitor Cri, VD3 is a terminal voltage of the diode D3, ID3 is a current of the diode D3, VD4 is a terminal voltage of the diode D4, and ID4 is a current of the diode D4.
There is a dead time during which the switching elements Q1 and Q2 are both OFF. The switching elements Q1 and Q2 are alternately turned on/off.
In an interval between t0 and t1, the switching element Q2 changes from ON to OFF at t0. In a state where the switching element Q2 is ON, the primary side of the transformer T1 passes a current in a clockwise direction through a path extending along Vin, Q2, Lr1, Lp1, Cri, and Vin and the secondary side of the transformer T1 passes a current through a path extending along Co1, Ro1, and Co1.
When the switching element Q2 turns off, the current of the primary side of the transformer T1 shifts from the switching element Q2 to the voltage resonant capacitor Crv and passes in a clockwise direction through a path extending along Crv, Lr1, Lp1, Cri, and Crv.
As a result, the voltage resonant capacitor Crv, which substantially has the voltage of the DC power source Vin in the state where the switching element Q2 is ON, discharges to 0 V (hereinafter, the voltage of the DC power source Vin is also represented with Vin).
Since the voltage of the voltage resonant capacitor Crv is equal to the voltage VQ1 of the switching element Q1, the voltage VQ1 of the switching element Q1 decreases from Vin to 0 V. The voltage VQ2 of the switching element Q2 is (Vin−VQ1), and therefore, increases from 0 V to Vin.
In an interval from t1 to t2, the voltage of the voltage resonant capacitor Crv decreases to 0 V at t1 and the diode D1 becomes conductive to pass a current in a clockwise direction through a path extending along D1, Lr1, Lp1 (P1), Cr1, and D1. The voltage of the secondary winding S2 of the transformer T1 reaches the output voltage Vo, and on the secondary side of the transformer T1, currents pass through the path extending along Co1, Ro1, and Co1 and a path extending along S2, D4, Co1, and S2. In the interval from t1 to t2, a gate signal to the switching element Q1 is set to ON so that the switching element Q1 conducts a zero-voltage switching (ZVS) operation and a zero-current switching (ZCS) operation.
In an interval from t2 to t3, the switching element Q1 is ON at t2 to pass a current in a counterclockwise direction through a path extending along Cri, Lp1 (P1), Lr1, Q1, and Cri, so that the voltage VCri of the current resonant capacitor Cri decreases. On the secondary side of the transformer T1, currents pass through the path extending along S2, D4, Co1, and S2 and the path extending along Co1, Ro1, and Co1. The voltage of the secondary winding S2 is clamped at the output voltage Vo and the voltage of the primary winding P1 is clamped at a voltage determined by the output voltage Vo and a turn ratio, so that the primary side of the transformer T1 passes a resonant current produced by the reactor Lr1 and current resonant capacitor Cri.
In an interval from t3 to t4, the voltage of the secondary winding S2 becomes lower than the output voltage Vo at t3 and the current on the secondary side of the transformer T1 becomes nil. On the secondary side of the transformer T1, a current passes through the path extending along Co1, Ro1, and Co1 in a clock wise manner. On the primary side of the transformer T1, a current passes in a counterclockwise direction through the path extending along Cri, Lp1, Lr1, Q1, and Cri. On the primary side of the transformer T1, a resonant current determined by the sum (Lr1+Lp1) of the two reactors Lr1 and Lp1 and the current resonant capacitor Cri are caused.
In an interval from t4 to t5, the switching element Q1 turns off at t4 and the current of the primary side of the transformer T1 shifts from the switching element Q1 to the voltage resonant capacitor Crv and passes in a counterclockwise direction through a path extending along Lp1, Lr1, Crv, Cri, and Lp1.
Accordingly, the voltage resonant capacitor Crv, which is about 0 V in the state in which the switching element Q1 is ON, is charged to Vin. Since the voltage of the voltage resonant capacitor Crv is equal to the voltage VQ1 of the switching element Q1, the voltage of the switching element Q1 increases from 0 V to Vin. The voltage VQ2 of the switching element Q2 is (Vin−VQ1), and therefore, decreases from Vin to 0 V.
In an interval from t5 to t6, the voltage of the voltage resonant capacitor Crv increases to Vin at t5 and the diode D2 becomes conductive to pass a current in a counterclockwise direction through a path extending along Lp1 (P1), Lr1, D2, Vin, Cri, and Lp1 (P1). Also, the voltage of the secondary winding S1 of the transformer T1 reaches the output voltage Vo, and on the secondary side of the transformer T1, currents pass through the path extending along Co1, Ro1, and Co1 and a path extending along S1, D3, Co1, and S1. In the interval from t5 to t6, a gate signal to the switching element Q2 is set to ON, so that the switching element Q2 conducts a zero-voltage switching operation and a zero-current switching operation.
In an interval from t6 to t7, the switching element Q2 turns on at t6 to pass a current in a clockwise direction through a path extending along Vin, Q2, Lr1, Lp1 (P1), Cri, and Vin and increase the voltage VCri of the current resonant capacitor Cri. On the secondary side of the transformer T1, currents pass through the path extending along S1, D3, Co1, and S1 and the path extending along Co1, Ro1, and Co1. The voltage of the secondary winding S1 is clamped at the output voltage Vo and the voltage of the primary winding P1 is clamped at a voltage determined by the output voltage Vo and a turn ratio. As a result, the primary side of the transformer T1 passes a resonant current produced by the reactor Lr1 and current resonant capacitor Cri.
In an interval from t7 to t8, the voltage of the secondary winding S1 becomes lower than the output voltage Vo at t7, and on the secondary side of the transformer T1, a current passes through the path extending along Co1, Ro1, and Co1. On the primary side of the transformer T1, a current passes in a clockwise direction through the path extending along Vin, Q2, Lr1, Lp1, Cri, and Vin. On the primary side of the transformer T1, a resonant current produced by the sum (Lr1+Lp1) of the two reactors Lr1 and Lp1 and the current resonant capacitor Cri are caused.
In this way, the DC conversion apparatus according to the related art illustrated in FIG. 1 employs a pulse signal having a duty of about 50% to control the switching frequency of the switching elements Q1 and Q2 and change the resonant current produced by the reactors Lr1 and Lp1 and current resonant capacitor Cri, thereby controlling the output voltage Vo. Namely, increasing the switching frequency results in decreasing the output voltage Vo.
The output smoothing means of the circuit illustrated in FIG. 1 is of a capacitor input type. To configure the secondary side of the transformer T1 so as to provide multiple outputs, secondary windings S11 and S12 are added to secondary windings S13 and S14 of a transformer T1a as illustrated in FIG. 2 and a voltage of the secondary windings S11 and S12 is rectified and smoothed. This may simply realize a multiple-output power source circuit. The secondary windings S11 and S12 and the secondary windings S13 and S14 are tightly coupled, and therefore, each output voltage is provided in proportion to a turn ratio, to achieve a proper cross regulation.
As mentioned above, the output voltage Vo on the secondary side of the transformer T1 is a voltage proportional to the number of turn, on secondary side, and therefore, the larger the number of turns on the secondary side of the transformer T1, the finer the output voltage is set.